Returns the additions to CKEDITOR.config for a specific CKEditor instance.
The editor's settings can be found in $editor->settings, but be aware that it may not yet contain plugin-specific settings, because the user may not yet have configured the form. If there are plugin-specific settings (verify with isset()), they can be found at $editor->settings['plugins'][$plugin_id].
\Drupal\editor\Plugin\Core\Entity\Editor $editor: A configured text editor object.
array A keyed array, whose keys will end up as keys under CKEDITOR.config.
public function getConfig(Editor $editor);